Recent reports from this laboratory have revealed that highly fluorinated 4- and 5-membered-ring enols are comparable in stability to, or more stable thermodynamically than, the corresponding ketones, even in non-Lewis-basic media. Work on perfluorinated keto-enol systems has now been extended to 2H-perfluorocyclohexanone plus its enol and to a series of acyclic analogues. In carbon tetrachloride, K-E/K = 0.33 (22 degrees C) for the six-ring system, but only enol is detectable in Lewis-basic solvents. This shift is attributable to strong hydrogen-bond formation between the enol and Lewis base. A perfluoroenol has been shown to form significantly stronger hydrogen bonds than the potent hexafluoroisopropyl alcohol. Acyclic systems (e.g., 3H-perfluoro-2-butanone and its enol) contrast sharply with the cyclic, as no enol is detectable at equilibrium even in powerfully Lewis-basic media. Ab initio quantum mechanical calculations indicate that it is principally the enols, not the ketones, that are responsible for the difference between the two types of keto-enol systems, i.e. acyclic perfluoroenols are strongly destabilized relative to cyclic counterparts.